Carbon fiber is employed as a fiber for reinforcing a composite material comprising a plastic usually called matrix resin owing to its excellent mechanical property, and is applied widely in various end uses including aerospace industry, sports goods industry, and other general industries.
A common method for manufacturing carbon fiber involves a process of producing precursor (also referred to as fiber production process), a process of converting the precursor into an oxidized fiber in an oxidative atmosphere at 200 to 300 deg.C. (hereinafter also referred to as oxidative stabilization process), and a process of carbonizing the oxidized fiber in an inert atmosphere at 300 to 2,000 deg.C. (hereinafter also referred to as carbonization process). The oxidative stabilization and carbonization processes are hereinafter also collectively referred to as baking process. The process of producing precursor includes a drawing step where acrylic fiber is drawn with a draw ratio higher than that for an ordinary acrylic fiber. At the drawing step, acrylic fiber is apt to adhere to adjacent fiber strands to be drawn unevenly under high draw ratio, and processed into nonuniform precursor. Such nonuniform precursor poses a problem, i.e., insufficient tenacity of resultant carbon fiber produced by baking the precursor. The baking process also poses another problem, i.e., fusing of single precursor fibers being baked, leading to reduced quality and grade of resultant carbon fiber.
For preventing the adhesion of single precursor fiber strands and fusing of carbon fiber, a number of techniques for applying finishes to precursors have been proposed ( see Japanese patent documents JP-A-60-181322 and JP-A-2001-172879), and widely used in industries. The techniques employ silicone finishes attaining low fiber-to-fiber wet friction in wet condition or at high temperature and excellent fiber detaching property, especially finishes comprising amino-modified silicones which cross-link on fiber to improve the heat resistance of the finish-applied fiber.
Amino-modified silicone finishes usually employed are aqueous emulsions of amino-modified silicone oils. A surfactant is employed for making an aqueous emulsion of an amino-modified silicone having no self-emulsification property. After applying such aqueous emulsion finish to precursor, the precursor fiber is fed to drying process for removing the water in the aqueous emulsion. Then the precursor is heated and drawn in drawing process to be processed into highly drawn precursor. Amino-modified silicones have excellent thermal cross-linking performance, and their cross-linking behavior is accelerated on heater rollers in drying and drawing processes to increase stain on the rollers (hereinafter also referred to as gumming up). The stain causes precursor breakage or fluffs on precursor, and decreases precursor production efficiency because of the work for cleaning the stain.
For solving such problems, finish formulae containing an antioxidant for inhibiting such gumming up have been proposed as in Japanese patent documents JP-A-2-91224 and JP-A-11-012853.